Ceria-alumina oxidation catalyst

ABSTRACT

Oxidation catalyst compositions-include a catalytic material containing ceria-and alumina each having a surface area of at least about 10 m 2  /g, for example, ceria and activated alumina in a weight ratio of from about 1.5:1 to 1:1.5. Optionally, platinum may be included in the catalytic material in amounts which are sufficient to promote gas phase oxidation of CO and HC but which are limited to preclude excessive oxidation of SO 2  to SO 3 . Alternatively, palladium in any desired amount may be included in the catalytic material. The catalyst compositions have utility as oxidation catalysts for pollution abatement of exhausts contianing unburned fuel or oil. For example, the catalyst compositions may be used in a method to treat diesel engine exhaust by contacting the hot exhaust with the catalyst composition to promote the oxidation of the volatile organic fraction component of particulates in the exhaust.

This application is a continuation of application Ser. No. 08/247,625,filed May 23, 1994 and now U.S. Pat. No. 5,462,907, which is acontinuation of Ser. No. 07/973,461, filed Nov. 19, 1992 and nowabandoned, which is a continuation-in-part of Ser. No. 07/798,437, filedNov. 26, 1991 and now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a catalyst composition and method for theoxidation of oxidizeable components of a gas-borne stream, e.g., for thetreatment of diesel engine exhaust, and more specifically to thetreatment of such diesel exhaust to reduce the particulates contentthereof.

2. Background and Related Art

As is well-known, gas-borne streams or engine exhausts often containoxidizeable pollutants such as unburned fuel and vaporized or condensedoils. For example, diesel engine exhaust contains not only gaseouspollutants such as carbon monoxide ("CO") and unburned hydrocarbons("HC"), but also soot particles which, as described in more detailbelow, comprise both a dry carbonaceous fraction and a hydrocarbonliquid which is sometimes referred to as a volatile organic fraction("VOF"), which terminology will be used herein, or a soluble organicfraction. Accordingly, although sometimes loosely referred to as an"exhaust gas", the exhaust of a diesel engine is actually aheterogeneous material, comprising gaseous, liquid and solid components.The VOF may exist in diesel exhaust either as a vapor or as an aerosol(fine droplets of liquid condensate) depending on the temperature of thediesel exhaust.

Oxidation catalysts comprising a platinum group metal dispersed on arefractory metal oxide support are known for use in treating the exhaustof diesel engines in order to convert both HC and CO gaseous pollutantsand particulates, i.e., soot particles, by catalyzing the oxidation ofthese pollutants to carbon dioxide and water. One problem faced in thetreatment of diesel engine exhaust is presented by the presence ofsulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide andthe oxidation catalyst catalyzes the SO₂ to SO₃ ("sulfates") withsubsequent formation of sulfuric acid. The sulfates also react withactivated alumina supports to form aluminum sulfates, which renderactivated alumina-containing catalysts inactive. In this regard, seeU.S. Pat. No. 4,171,289 at column 1, line 39 et seq. Previous attemptsto deal with the sulfation problem include the incorporation of largeamounts of sulfate-resistant materials such as vanadium oxide into thesupport coating, or the use of alternative support materials such asα-alumina, silica and titania, which are sulfation-resistant materials.Further, as is known, the oxidation of SO₂ to SO₃ also adds to theparticulates in the exhaust by forming condensible sulfur compounds,such as sulfuric acid, which condense upon, and thereby add to, the massof particulates.

Generally, the prior art has attempted to deal with these problems bydispersing a suitable oxidation catalyst metal, such as one or moreplatinum group metals, upon a refractory metal oxide support which isresistant to sulfation.

Examples of catalysts designed for the treatment of diesel exhaust fumesand soot include U.S. Pat. No. 4,849,399 to Joy et al dated Jul. 18,1989. This patent discloses catalytic composites which incorporatesulfur-resistant refractory inorganic oxides selected from the groupconsisting of titania, zirconia, and alumina treated with titania and/orzirconia (see column 6, lines 62-68).

U.S. Pat. No. 4,759,918 to Homeier et al dated Jul. 26, 1988 disclosescatalysts for the treatment of diesel exhaust fumes and soot whichincorporate sulfur-resistant refractory inorganic oxides selected from agroup which includes silica, alumina, and silica-alumina (see column 3,lines 16-27).

SUMMARY OF THE INVENTION

Generally, in accordance with the present invention, there is providedan oxidation catalyst composition and a method for oxidizing oxidizeablecomponents of a gas-borne stream, e.g., for treating diesel engineexhaust in which at least a volatile organic fraction component(described below) of the diesel exhaust particulates is converted toinnocuous materials, and in which gaseous HC and CO pollutants may alsobe similarly converted. The objectives of the invention are attained byan oxidation catalyst comprising a base metal oxide catalytic materialconsisting essentially of a mixture of high surface area ceria and highsurface area alumina, which optionally may have dispersed thereon a lowloading of platinum catalytic metal. The method of the invention isattained by flowing a gas-borne stream, e.g., a diesel engine exhaust,into contact under reaction conditions with a catalyst composition asdescribed above. In the case of treating diesel exhaust, the exhaust maybe contacted under reaction conditions with a catalyst composition whichcontains palladium instead of a low loading of platinum but is otherwiseas described above.

Specifically, in accordance with the present invention there is providedan oxidation catalyst composition which comprises a refractory carrieron which is disposed a coating of a ceria-alumina catalytic materialconsisting essentially of a combination of ceria and alumina each havinga BET surface area of at least about 10 m² /g, preferably the aluminahaving a surface area of from about 25 m² /g to 200 m² /g and the ceriahaving a surface area of from about 25 m² /g to 200 m² /g.

In one embodiment of the invention, the ceria and alumina each comprisesfrom about 5 to 95 percent, preferably from about 10 to 90 percent, morepreferably from about 40 to 60 percent, by weight of the combination.

One aspect of the invention provides that the catalyst compositionoptionally further comprises a catalytically effective amount ofplatinum dispersed on the catalytic material in an amount not to exceedabout 15 g/ft³ of the catalyst composition. For example, the platinummay be present in the amount of from about 0.1 to 15 g/ft³ of thecomposition, preferably from about 0.1 to 5 g/ft³ of the composition.When the catalyst composition includes platinum, another aspect of theinvention provides that at least a catalytically effective amount of theplatinum is dispersed on the ceria. At least a catalytically effectiveamount of the platinum may also be dispersed on the alumina. Suchdispersal of the platinum may be utilized whether the alumina and ceriaare mixed in a single layer or are present in discrete layers of,respectively, ceria and alumina and, in the latter case, irrespective ofwhich of the two layers is the top layer.

Still another aspect of the invention provides that the ceria comprisesan aluminum-stabilized ceria. The alumina may also be stabilized againstthermal degradation.

The ceria and alumina may be combined as a mixture and the mixturedeposited as a single layer coating on the refractory carrier, or theceria and alumina may be present in respective discrete superimposedlayers of ceria and alumina. The ceria layer may be above or below thealumina layer.

In accordance with the method of the present invention, there isprovided a method of treating diesel engine exhaust containing avolatile organic fraction. The method includes contacting the exhaustwith a catalyst composition comprised of components as described aboveor with a catalyst composition comprised of components as describedabove but which optionally includes palladium instead of the optionalplatinum. Thus, the method includes contacting the gas-borne stream tobe treated with a catalyst composition comprising ceria and alumina asdescribed above, and optionally including platinum or palladium. Whenthe optional palladium is employed in the composition, it may be presentin the amount from about 0.1 to 200 g/ft³, preferably in the amount offrom about 20 to 120 g/ft³, of the catalyst composition. In accordancewith the method of the present invention, contacting of the dieselexhaust with the catalyst composition is carried out at a temperaturehigh enough to catalyze oxidation of at least some of the volatileorganic fraction of the exhaust, for example, an inlet temperature offrom about 100° C. to 800° C.

Definations

As used herein and in the claims, the following terms shall have theindicated meanings.

The term "gas-borne stream" means a gaseous stream which may containnon-gaseous components such as solid particulates and/or vapors, liquidmist or droplets, and/or solid particulates wetted by a liquid.

The term "BET surface area" has its usual meaning of referring to theBrunauer, Emmett, Teller method for determining surface area by N₂adsorption. Unless otherwise specifically stated, all references hereinto the surface area of a ceria, alumina or other component refer to theBET surface area.

The term "activated alumina" has its usual meaning of a high BET surfacearea alumina, comprising primarily one or more of γ-, θ- and δ-aluminas(gamma, theta and delta).

The term "catalytically effective amount" means that the amount ofmaterial present is sufficient to affect the rate of reaction of theoxidation of pollutants in the exhaust being treated.

The term "inlet temperature" shall mean the temperature of the exhaust,test gas or other stream being treated immediately prior to initialcontact of the exhaust, test gas or other stream with the catalystcomposition.

The term "ceria-alumina catalytic material" means a combination of ceriaparticles and alumina particles each having a BET surface area of atleast about 10 m² /g, i.e., a combination of high surface area bulkceria and high surface area bulk alumina, sometimes referred to as"activated alumina".

The term "combination" when used with reference to a combination ofceria and alumina includes combinations attained by mixtures or blendsof ceria and alumina as well as superimposed discrete layers of ceriaand alumina.

The term "aluminum-stabilized ceria" means ceria which has beenstabilized against thermal degradation by incorporation therein of analuminum compound. A suitable technique is shown in U.S. Pat. No.4,714,694 of C. Z. Wan et al (the disclosure of which is incorporated byreference herein), in which ceria particles are impregnated with aliquid dispersion of an aluminum compound, e.g., an aqueous solution ofa soluble aluminum compound such as aluminum nitrate, aluminum chloride,aluminum oxychloride, aluminum acetate, etc. After drying and calciningthe impregnated ceria in air at a temperature of, e.g., from about 300°C. to 600° C. for a period of 1/2 to 2 hours, the aluminum compoundimpregnated into the ceria particles is converted into an effectivethermal stabilizer for the ceria. The term "aluminum-stabilized" is usedfor economy of expression although the aluminum is probably present inthe ceria as a compound, presumably alumina, and not as elementalaluminum.

Reference herein or in the claims to ceria or alumina being in "bulk"form means that the ceria or alumina is present as discrete particles(which may be, and usually are, of very small size, e.g., 10 to 20microns in diameter or even smaller) as opposed to having been dispersedin solution form into another component. For example, the thermalstabilization of ceria particles (bulk ceria) with alumina as describedabove with respect to U.S. Pat. No. 4,714,694 results in the aluminabeing dispersed into the ceria particles and does not provide thedispersed alumina in "bulk" form, i.e., as discrete particles ofalumina.

The abbreviation "TGA" stands for thermogravimetric analysis which ismeasure of the weight change (e.g., loss) of a sample as a function oftemperature and/or time. The abbreviation "DTA" stands for differentialthermal analysis which is measure of the amount of heat emitted(exotherm) or absorbed (endotherm) by a sample as a function oftemperature and/or time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of oxidation of SO₂ to SO₃ in a gas stream beingtreated with an oxidation catalyst, the degree of oxidation beingplotted on the ordinate versus the platinum loading of the catalyst onthe abscissa;

FIG. 2 is a plot similar to that of FIG. 1 but showing the degree of HCoxidation on the ordinate versus platinum loading on the abscissa;

FIG. 3 is a perspective plot of oxidation of SO₂ to SO₃ in a gas streambeing treated with an oxidation catalyst, with the degree of oxidationindicated by the height of the vertical bars for three differentsamples, each containing 0.5 g/ft³ of platinum and having differentweight percentages of ceria in the ceria-alumina catalytic material;

FIG. 4 is a plot of a factor (DTA peak area) correlating combustion ofengine lubricating oil (simulating the unburned lubricating oil in"VOF", described below) plotted on the ordinate versus the platinumcontent of a ceria-alumina washcoat used to catalyze the combustion ofthe lubricating oil plotted on the abscissa; and

FIGS. 5 through 8 are plots showing various aspects of diesel engineexhaust treatment performance of three aged catalyst samples made inaccordance with certain embodiments of the present invention as afunction of the operating temperature of the catalysts, as follows: FIG.5 shows the percentage conversion of the volatile organic fraction("VOF"); FIG. 6 shows the percentage conversion of total particulatematter ("TPM") in the exhaust; FIG. 7 shows the gas phase conversion ofhydrocarbons ("HC") and FIG. 8 shows the gas phase conversion of carbonmonoxide ("CO").

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF

The present invention provides an oxidation catalyst composition whichis effective for oxidizing oxidizeable components of a gas-borne stream,for example, for treating diesel engine exhaust. In the latter case, thecomposition is particularly effective with regard to reducing the totalparticulates in the exhaust. The carbonaceous particulates ("soot")component of diesel engine exhaust is, as is well-known, comprised oftwo major components. One component is relatively dry carbonaceousparticles and the other, usually referred to as a volatile organicfraction ("VOF"), is a mixture of high molecular weight hydrocarbonscomprised of unburned and partially burned diesel fuel and lubricatingoil. The volatile organic fraction is present in the diesel exhaust aseither a vapor phase or a liquid phase, or both, depending on thetemperature of the exhaust. Generally, it is not feasible to attempt toremove or treat the dry, solid carbonaceous particulates component ofthe total particulates by catalytic treatment, and it is the VOFcomponent which can be most effectively removed by conversion viautilization of an oxidation catalyst. Therefore, in order to reduce thetotal particulates discharged so as to meet present and impendingGovernment regulations concerning maximum allowable total particulates,the volatile organic fraction, or at least a portion thereof, isoxidized to innocuous CO₂ and HO₂ by being contacted with an oxidationcatalyst under suitable reaction conditions. The required U.S.Government limits for 1991 on HC, CO, nitrogen oxides ("NO_(x) ") andtotal particulate emissions ("TPM") in diesel engine exhaust have beenlargely met by suitable engine design modifications. For 1994 the HC, COand NO_(x) limits remain unchanged from 1991 standards but the upperlimit on TPM will be reduced from the 1991 level of 0.25 grams perhorsepower-hour ("g/HP-hr") to 0.10 g/HP-hr. Although the oxidationcatalysts of the present invention, when employed as a diesel exhausttreatment catalyst, are primarily concerned with effectuating areduction in total particulates, they are also capable, with theoptional addition of platinum in limited amounts of providing the addedadvantage of also oxidizing a portion of the HC and CO contained in thegaseous component of the diesel engine exhaust without promotingexcessive oxidation of SO₂ to SO₃. The oxidation catalysts of thepresent invention avoid or reduce the unwanted side effect of promotingthe oxidation of SO₂ to SO₃ which, as noted above, contributes to theparticulates problem because the condensation of sulfuric acid and othersulfate condensibles which accumulate on, and add to, the mass of theparticulates in the exhaust.

However, the oxidation catalysts of the present invention have utilityfor uses other than the treatment of diesel engine exhaust. Generally,the catalysts of the present invention are useful for oxidation ofgas-borne oxidizeable components in engine exhausts generally, such asany application in which lubricating oils are discharged, e.g., theexhaust of compressed natural gas engines, ethanol-fueled engines,compressors, gas turbines, etc. Many alternate-fueled engines such ascompressed natural gas engines are built on diesel engine carcasses andtherefore inherently discharge significant quantities of lubricatingoil.

In accordance with the teachings of the present invention it has beenfound, surprisingly, that the beneficial effect of oxidizing pollutantsgenerally, and in particular of reducing diesel exhaust particulatesemissions by oxidation of the volatile organic fraction thereof, can beattained by a mixture of high surface area, i.e., activated, alumina anda high surface area ceria, each having a BET surface area of 10 m² /g orhigher. For purposes of illustration, the benefits of the presentinvention will be described in detail below with respect to thetreatment of diesel engine exhaust. The basic and novel characteristicsof the present invention are believed to reside in the use of thedefined combination of ceria and alumina as an oxidation catalystwithout the addition of metal catalytic components thereto, except asspecifically otherwise defined in certain dependent claims. Preferably,the bulk ceria and the bulk alumina will each have a surface area of atleast about 10 m² /g, preferably at least about 20 m² /g. For example,the bulk alumina may have a surface area of from about 120 to 180 m² /gand the bulk ceria may have a surface area of from about 70 to 150 m²/g. The fact that a catalyst composition which can serve as a dieseloxidation catalyst and which contains activated alumina as a majorcomponent thereof has proven to be successful is in itself surprising,in view of the consensus of the prior art that alumina, if used at allin diesel oxidation catalysts, must be a low surface area alumina(α-alumina) and/or be used in conjunction with sulfate-resistantrefractory metal oxides such as zirconia, titania or silica. It hasnonetheless been found that, in accordance with the present invention,surprisingly, a combination of high surface area alumina and a highsurface area ceria provides a catalytic material which effectivelycatalyzes the oxidation of the volatile organic fraction so as toprovide a significant reduction in total particulates in diesel engineexhaust and exhibits good durability, that is, long life, both inlaboratory and diesel engine tests. It should be noted that the priorart generally considers refractory base metal oxides used in dieseloxidation catalysts to be merely supports for the dispersal thereon ofcatalytically active metals such as platinum group metals. In contrast,the present invention teaches that a ceria-alumina catalytic materialcomprising essentially only ceria and alumina of sufficiently highsurface area (10 m² /g or higher), dispersed on a suitable carrier,provides a durable and effective diesel oxidation catalyst.

It has further been found that beneficial effects are attained by theoptional incorporation of platinum in the catalyst composition, providedthat the platinum is present at loadings much lower than thoseconventionally used in oxidation catalysts. It has been discovered that,most surprisingly, a limited quantity of platinum in the catalystcomposition actually reduces the undesirable oxidation of SO₂ to SO₃relative to that encountered by using the ceria-alumina catalyticmaterial alone, while nonetheless promoting some oxidation of CO and HCgaseous components of the diesel exhaust. The suppression of theoxidation of SO₂ to SO₃ by the addition of low loadings of platinum is avery surprising finding, given the powerful catalytic activity ofplatinum in promoting oxidation reactions generally. Without wishing tobe bound by any particular theory, it may be that the presence of a lowloading of platinum on the ceria occupies some catalytic sites on theceria, thereby moderating the tendency of ceria to promote the oxidationof SO₂ to SO₃. If the catalytic metal platinum is added to the catalyticcomposition, it serves to catalyze the oxidation of gas phase HC and COpollutants as an added benefit. However, such catalytic metal is notneeded to supplement the action of the ceria-alumina catalytic materialin reducing total particulate emissions. The platinum catalytic metaldoes not appear to play a role in controlling particulates, as indicatedby data discussed elsewhere herein, which show that the quantity ofplatinum utilized does not significantly affect the rate of particulatesconversion.

The catalysts of the present invention may take the form of a carrier orsubstrate, such as a monolithic "honeycomb" structure (a body having aplurality of gas flow passages extending therethrough), on which isapplied a coating of the catalytic material comprising a mixture of highsurface area ceria and alumina and, optionally, a low loading platinum.As discussed below, discrete coatings of the ceria and alumina may beemployed.

The Carrier (Substrate)

The carrier used in this invention should be relatively inert withrespect to the catalytic composition dispersed thereon. The preferredcarriers are comprised of ceramic-like materials such as cordierite,α-alumina, silicon nitride, zirconia, mullite, spodumene,alumina-silica-magnesia or zirconium silicate, or of refractory metalssuch as stainless steel. The carriers are preferably of the typesometimes referred to as honeycomb or monolithic carriers, comprising aunitary cylindrical body having a plurality of fine, substantiallyparallel gas flow passages extending therethrough and connecting bothend-faces of the carrier to provide a "flow-through" type of carrier.Such monolithic carriers may contain up to about 700 or more flowchannels ("cells") per square inch of cross section, although far fewermay be used. For example, the carrier may have from about 7 to 600, moreusually from about 200 to 400, cells per square inch ("cpsi").

While this discussion and the following examples relate to flow-throughtype carrier substrates, wall-flow carriers (filters) may also be used.Wall-flow carriers are generally similar in structure to flow-throughcarriers, with the distinction that each channel is blocked at one endof the carrier body, with alternate channels blocked at oppositeend-faces. Wall-flow carrier substrates and the support coatingsdeposited thereon are necessarily porous, as the exhaust must passthrough the walls of the carrier in order to exit the carrier structure.

The Catalytic Material

The ceria-alumina catalytic material may be prepared in the form of anaqueous slurry of ceria and alumina particles, the particles optionallybeing impregnated with the platinum catalytic metal component if one isto be utilized. The slurry is then applied to the carrier, dried andcalcined to form a catalytic material coating ("washcoat") thereon.Typically, the ceria and alumina particles are mixed with water and anacidifier such as acetic acid, nitric acid or sulfuric acid, and ballmilled to a desired particle size.

The optional platinum catalytic metal component is, when used,preferably incorporated into the ceria particles or into the ceria andalumina particles. In such case, the ceria-alumina acts not only as acatalyst but also as a support for the optional platinum catalytic metalcomponent. Such incorporation may be carried out after the ceria-aluminacatalytic material is coated as a wash-coat onto a suitable carrier, byimpregnating the coated carrier with a solution of a suitable platinumcompound, followed by drying and calcination. However, preferably, theceria particles or both the ceria and alumina particles are impregnatedwith a suitable platinum compound before a coating of the ceria-aluminacatalytic material is applied to the carrier. In either case, theoptional platinum metal may be added to the ceria-alumina catalyticmaterial as, e.g., a solution of a soluble platinum compound, thesolution serving to impregnate the ceria and alumina particles (or theceria-alumina coating on the carrier), which may then be dried and theplatinum fixed thereon. Fixing may be carried out by calcination or bytreatment with hydrogen sulfide or by other known means, to render themetal in water-insoluble form.

Generally, the slurry of ceria and activated alumina particles, whetheror not impregnated with the platinum compound solution, will bedeposited upon the carrier substrate and dried and calcined to adherethe catalytic material to the carrier and, when the platinum compound ispresent, to revert the platinum compound to the elemental metal or itsoxide. Suitable platinum compounds for use in the foregoing processinclude potassium platinum chloride, ammonium platinum thiocyanate,amine-solubilized platinum hydroxide and chloroplatinic acid, as iswell-known in the art. During calcination, or at least during theinitial phase of use of the catalyst, such compounds, if present, areconverted into the catalytically active elemental platinum metal or itsoxide.

When the catalytic material is applied as a thin coating to a suitablecarrier, such as described above, the proportions of ingredients areconventionally expressed as weight of material per unit volume ofcatalyst, as this measure accommodates the presence of different sizesof catalyst composition voids provided by different carrier wallthicknesses, gas flow passages, etc. Grams per cubic inch ("g/in³ ")units are used to express the quantity of relatively plentifulcomponents such as the ceria-alumina catalytic material, and grams percubic foot ("g/ft³ ") units are used to express the quantity of thesparsely used ingredients, such as the platinum metal. For typicaldiesel exhaust applications, the ceria-alumina catalytic material of thepresent invention generally may comprise from about 0.25 to about 4.0g/in³, preferably from about 0.25 to about 3.0 g/in³ of the coatedcarrier substrate, optionally including from about 0 to 25, preferablyfrom about 0 to 15 g/ft³ of platinum.

Without wishing to be bound by a particular theory, applicants offer thefollowing hypothesis to explain the superior performance, when used totreat diesel engine exhaust, of the ceria-alumina catalytic materialsaccording to this invention. It is believed that diesel exhaust containsa significant proportion of gases or vapors which are close to their dewpoint, i.e., close to condensing to a liquid, and thereby adding to theVOF portion of the particulates at the conditions obtaining in theexhaust pipe. These "potential particulates" condense in theceria-alumina catalytic materials, their condensation being enhanced bya capillary condensation effect, a known phenomenon in which acapillary-like action facilitates condensation of oil vapors to liquidphase. The small pore size of the high surface area ceria-aluminacatalytic material is believed to provide such capillary condensationaction for the VOF. Generally, the higher the surface area of the ceriaand alumina, the smaller is their pore size. As the exhaust temperatureincreases during increased work loads imposed on the diesel engine, thecondensed hydrocarbon liquids (condensed VOF) are desorbed from theceria-alumina catalytic material and volatilize, at which time thecatalytic effect of the ceria-alumina catalytic material, which providesnumerous acidic sites, is believed to enhance cracking and gas phaseoxidation, i.e., combustion, of the desorbed, re-volatilized hydrocarbon(VOF) vapors. Even if a proportion of the vapors re-volatilized from thecondensate is not combusted, the cracking of heavy VOF components tolighter hydrocarbons reduces the total amount of condensibles, so thatthe total particulates output from the diesel engine is concomitantlyfurther reduced. In this latter regard, the ceria-alumina catalyticmaterial is believed to act as a trap and a storage medium for condensedor condensible VOF during relatively cool phases of the exhaust, andreleases the cracked VOF only upon re-volatilization thereof duringrelatively hot phases. The porous nature of the ceria-alumina catalyticmaterial is also believed to promote rapid diffusion of the VOFthroughout the washcoat structure, thereby facilitating relatively lowtemperature gasification and oxidation of the VOF upon increases intemperature of the catalyst during higher engine load (and thereforeincreased exhaust gas temperature) cycles. Data on aging show that thepresence of sulfates does not significantly adversely affect thecapacity of the ceria-alumina catalytic material to reduce particulateemissions.

Generally, other ingredients may be added to the catalyst composition ofthe present invention such as conventional thermal stabilizers for thealumina, e.g., rare earth metal oxides such as ceria. Thermalstabilization of high surface area ceria and alumina to militate againstphase conversion to less catalytically effective low surface area formsis well-known in the art although thermal stabilization of alumina isnot usually needed for diesel exhaust service. Such thermal stabilizersmay be incorporated into the bulk ceria or into the bulk activatedalumina, by impregnating the ceria (or alumina) particles with, e.g., asolution of a soluble compound of the stabilizer metal, for example, analuminum nitrate solution in the case of stabilizing bulk ceria. Suchimpregnation is then followed by drying and calcining the impregnatedceria particles to convert the aluminum nitrate impregnated therein intoalumina.

In addition, the catalyst compositions of the invention may containother catalytic ingredients such as other base metal promoters or thelike. However, in one embodiment, the catalyst composition of thepresent invention consists essentially only of the high surface areaceria and high surface area alumina, preferably present in a weightproportion of 1.5:1 to 1:1.5, with or without thermal stabilizersimpregnated therein, and, optionally, limited amounts of platinum. Withrespect to the method aspect of the invention, the use of palladium inplace of platinum is contemplated.

Examples and Data

A catalyst composition in accordance with one embodiment of theinvention, in which an optional alumina undercoat is provided beneath acoating of the ceria-alumina catalytic material having a platinum metaldispersed thereon, was prepared as follows.

EXAMPLE 1

A. An activated alumina undercoat slurry is prepared by combining 1000grams of activated alumina having a nominal BET surface area of 150 m²/g with 50 cubic centimeters ("cc") of glacial acetic acid and 1 cc ofan antifoamant sold under the trademark NOPCO NXZ in 1000 cc ofdeionized water. The ingredients are ball milled until an averageparticle size of at least 90 percent by volume of the particles having adiameter of not greater than 12 microns is attained. Cylindricalcarriers comprising cordierite cylinders 6 inches long by 6 inches indiameter and having 400 gas flow passages per square inch of end facearea (400 cpsi) are dipped into the slurry, excess slurry is blown fromthe gas flow passages and the slurry-coated carriers are dried at 110°C. and then calcined in air at 450° C. for 1 hour to providealumina-coated carriers.

B. The ceria-alumina catalytic material is prepared by utilizing 1050grams of the same activated alumina as used in Part A and 900 grams ofaluminum-stabilized ceria having a BET surface area of 164 m² /g. Thealuminum-stabilized ceria is attained by impregnating the ceriaparticles with a solution of an aluminum compound such as aluminumnitrate followed by calcining, to provide an aluminum content in theceria of 1.35 weight percent aluminum, based on the total weight ofceria with the weight of aluminum calculated as the metal. Presumably,the aluminum is present as alumina. One such method of preparing analuminum-stabilized ceria is shown in U.S. Pat. No. 4,714,694 issuedDec. 22, 1981 to C. Z. Wan et al, the disclosure of which, as notedabove, is incorporated by reference herein. As is well-known, highsurface area refractory oxides such as ceria are subject to loss ofsurface area and consequent reduction in catalytic efficiency uponprolonged exposure to high temperatures and other conditions of treatingdiesel exhausts.

Aluminum-stabilized ceria is more resistant to such thermal degradationthan is unstabilized ceria. As is also well-known, alumina may also bethermally stabilized, usually by a similar impregnation of the aluminawith precursors of rare earth metal oxides such as ceria. However,thermal stabilization of the alumina is usually not necessary for thetemperatures encountered in treating diesel engine exhaust. The highsurface area ceria and high surface area alumina particles are placed inseparate ball mills. A quantity of an amine-solubilized platinumhydroxide solution containing 0.2894 grams of platinum, a quantity ofmonoethanolamine ("MEA"), 97.5 cc of glacial acetic acid, 2.0 cc of ananti-foamant sold under the trademark NOPCO NXZ and about 1950 cc ofdeionized water are employed. About one-half the water and sufficientMEA to adjust the pH to at least about 7 are placed in the ball millcontaining the alumina which is milled to thoroughly blend theingredients. Then, one-half of the platinum solution is added and ballmilling is continued for about 5 minutes. Thereafter, about one-half theglacial acetic acid and anti-foamant are added and milling is continueduntil a particle size of at least about 90 percent by weight of theparticles having a diameter of less than about 12 microns is attained.The same process is separately repeated with the aluminum-stabilizedceria, except that MEA is not employed, including ball milling formixing and to attain the same particle size of the ceria particles. Thealumina and ceria slurries are then blended together to form a slurry ofalumina and ceria particles containing a platinum compound. Thealumina-coated carrier obtained in Part A of this Example 1 is dippedinto the blended slurry, excess slurry is blown from the gas flowpassages of the carrier, and the coated carrier is then dried andcalcined in air at 450° C. to provide a finished catalyst containing acoating of a ceria-alumina catalytic material having about 0.5 g/ft³ ofplatinum dispersed thereon. The catalytic material coating, sometimesreferred to as a washcoat, inclusive of the platinum content, comprisesabout 1.95 g/in³ of the catalyst composition, the catalytic materialoverlying an alumina undercoat which comprises about 1.00 g/in³ of thecatalyst composition. Unless otherwise specified, catalyst samples inaccordance with the present invention in subsequent Examples have thesame type and loading of alumina undercoat and ceria-alumina catalyticmaterial as a topcoat overlying the undercoat.

Reference in the following TABLES, or elsewhere in this application, toa percentage conversion of constituents (rendered as "%C" in the TABLES)of the exhaust or test gas, means the percentage of such constituentinitially present in the exhaust or test gas being treated which isconverted to another species, e.g., the conversion to H₂ O and/or CO₂ ofHC, CO and VOF, and the oxidation to SO₃ of SO₂. Thus, if an exhaustcontains 10 volume percent CO and treatment of the exhaust results in anoutlet gas containing 6 volume percent CO, a 40 percent conversion ofthe CO has been attained. Reference in the following Examples, orelsewhere in this application, to "space velocity" means the flow rateof exhaust or test gas flowed through a catalyst, expressed as volumesof exhaust or test gas per volume of catalyst per hour, calculated withthe exhaust or test gas at standard conditions of temperature andpressure.

EXAMPLE 2

A series of sample catalysts was prepared generally in accordance withthe procedures of Example 1 to provide a series of five otherwiseidentical compositions containing a ceria-alumina catalytic material inaccordance with the teachings of the present invention, having variousamounts of platinum dispersed thereon, including 0, 0.5, 1.0, 2.0 and5.0 g/ft³ of platinum. These catalyst samples comprised cores measuring1.5 inches in diameter and 3.0 inches in length, cut from cordieritecarriers 6 inches long and 6 inches in diameter, used, as in Example 1,to make the catalysts of this Example 2. The resulting 400 cpsicordierite sample cores contained a loading of 1.95 g/in³ of theceria-alumina catalytic material overlying an alumina undercoat presentin the amount of 1.00 g/in³ in addition to the specified loading ofplatinum metal dispersed on the ceria-alumina catalytic material. Thetest catalysts were aged for 10 hours at 500° C. by having a mixture of10 percent steam in air flowed through them. A test gas was contactedwith each of these aged catalysts in a series of tests at a spacevelocity of 50,000 and inlet temperature of, respectively, 275° C., 350°C., 425° C. and 500° C. The test gas had a composition of 10 percentsteam, 10 percent oxygen, 4.5 percent CO₂, 1000 ppm NO, 28.6 ppmheptane, 200 ppm CO, 50 ppm SO₂, balance nitrogen. All percents arevolume percent and "ppm" means parts per million by volume. Measurementswere taken to determine the amount of oxidation of SO₂ to SO₃. Theresults of these tests are tabulated in TABLE I below and plotted inFIG. 1.

                  TABLE I                                                         ______________________________________                                        Inlet Gas                                                                              Platinum    % C.sup.a % C.sup.a                                                                           % C.sup.a                                Temp. (°C.)                                                                     Loading (g/ft.sup.3)                                                                      SO.sub.2  HC    CO                                       ______________________________________                                        275      0           8.0       0.0   0.0                                      275      0.5         0.0       2.4   30.5                                     275      1.0         6.1       0.0   74.6                                     275      2.0         16.0      10.0  99.0                                     275      5.0         30.6      20.2  99.5                                     350      0           8.0       0.0   5.9                                      350      0.5         4.0       9.8   68.3                                     350      1.0         17.6      31.7  97.9                                     350      2.0         21.6      87.8  100                                      350      5.0         30.0      83.1  100                                      425      0           12.0      2.6   10.3                                     425      0.5         11.8      31.6  84.3                                     425      1.0         25.5      66.6  96.4                                     425      2.0         33.3      90.5  100                                      425      5.0         48.0      91.9  100                                      500      0           20.0      9.3   9.3                                      500      0.5         12.0      47.4  84.8                                     500      1.0         28.8      80.5  98.5                                     500      2.0         35.3      83.1  99.5                                     500      5.0         62.0      88.0  100                                      ______________________________________                                         .sup.a "% C" means the percentage conversion of the indicated constituent                                                                              

The data of TABLE I, and the plot thereof in FIG. 1, clearly show thatthe ceria-alumina catalytic material containing no platinum in each caseprovided, at each temperature level tested, a somewhat higher degree ofconversion of SO₂ to SO₃ than did the otherwise identical ceria-aluminacatalyst containing 0.5 g/ft³ of platinum. As the platinum loading wasincreased to 1.0 g/ft³, at each temperature level, the degree ofundesired conversion of SO₂ to SO₃ increased as compared to the versionscontaining no or only 0.5 g/ft³. Further increases in platinum loadingto 2 and 5 g/ft³ further increased, as one would expect, the oxidationof SO₂. What is very surprising is the fact, clearly shown in FIG. 1 andthe data of TABLE I, that the ceria-alumina catalytic materialcontaining 0.5 g/ft³ of platinum dispersed thereon demonstrated lessconversion of SO₂ to SO₃ than did the ceria-alumina catalytic materialcontaining no platinum metal thereon. As noted above, it is believedthat the presence of a low loading of platinum on the ceria may occupysome catalytic sites which otherwise are highly effective in promotingthe oxidation of SO₂ to SO₃.

FIG. 2 shows the corresponding conversion of hydrocarbons in the testgas at the various temperature levels tested. The HC and CO conversiondata of TABLE I, and the plot of the HC conversion data of TABLE I inFIG. 2, show the expected result that as the content of platinum metalincreases the degree of conversion of HC and CO likewise increases. Asdiscussed elsewhere herein, because of successful modifications indiesel engine design, catalytic treatment of diesel exhaust may not benecessary in order to attain reductions in HC and CO to meet U.S.Government standards, because the modified engines have reduced theoutput of HC and CO to below that of the current and impending U.S.Government standards. Nonetheless, the inclusion of platinum, at leastat a loading of not more than about 1 g/ft³, preferably at from about0.1 to 0.8 g/ft³, more preferably at about 0.5 g/ft³, is seen to have abeneficial effect on reducing the amount of oxidation of SO to SO. Thuslimited, the addition of platinum is seen to reduce SO₂ oxidation andthereby ameliorate particulates emissions. The addition of platinum alsoprovides a beneficial added effect of further reducing HC and COemissions.

It will be appreciated that in some cases it may be desired or necessaryto significantly reduce HC and/or CO emissions and, in order to do so,the addition of moderate amounts of platinum, not more than 15 g/ft³,preferably not more than 5 g/ft³, and most preferably not more than 2g/ft³, may be desirable despite the concomitant increase in SO₂oxidation at additions of significantly more than 0.5 g/ft³.

EXAMPLE 3

A series of test catalysts was prepared generally in accordance with theprocedure outlined in Example 1 to provide three samples, eachcomprising an alumina undercoat at a loading of 1.0 g/in³ upon which wascoated a topcoat layer comprised of a ceria-alumina catalytic materialcontaining ceria and alumina in proportions of 46.2 weight percentaluminum-stabilized ceria and 53.8 weight percent alumina, and havingdispersed thereon 0.5 g/ft³ of platinum. The ceria-alumina topcoat layerwas present in the amount of 1.95 g/in³. The ceria had a surface area ofabout 164 m² /g and the alumina had a surface area of about 150 m² /g.One sample, designated S-3Ce, has the platinum dispersed only on theceria component of the catalytic material, a second sample designatedS-3 has equal amounts of the platinum dispersed on the ceria and thealumina components of the ceria-alumina catalytic material, and thethird sample, designated S-3A1, has the platinum disposed entirely onthe alumina component of the ceria-alumina catalytic material. The threecatalyst samples were then tested for HC, CO and SO₂ conversion at 350°C. and a space velocity of 90,000. The results are shown in TABLE IIbelow.

                  TABLE II                                                        ______________________________________                                                 % C.sup.a     % C.sup.a                                                                             % C.sup.a                                      Sample   CO            HC      SO.sub.2                                       ______________________________________                                        S-3Ce    80.2          37.5    4.1                                            S-3      49.3          7.55    4.6                                            S-3Al    94.9          56.5    8.0                                            ______________________________________                                         .sup.a "% C" means the percentage conversion of the indicated constituent                                                                              

The data of TABLE II clearly indicate that the platinum is a moreeffective oxidation catalyst for HC and CO when dispersed on the alumina(S-3A1) as compared to when it is dispersed on the ceria (S-3Ce) and ismuch more effective in this regard than is the S-3 sample, wherein theplatinum is dispersed equally on each of the ceria and aluminacomponents. Overall, the best results were obtained with the S-3Cesample in which fairly high levels of desired conversion of CO and HCwere attained and the lowest level (4.1%) of the undesired oxidation ofSO₂ to SO₃ was also attained. S-3 catalyst provided significant, butlesser, conversions of CO and HC and only slightly more (4.6%) of theundesired oxidation of SO₂ than did S-3Ce, but was much better in termsof less promotion of oxidation of SO₂ than was the S-3A1 sample (8.0%).TABLE II thus demonstrates the desirability of dispersing all or atleast a part of the platinum metal component on the ceria component ofthe ceria-alumina catalytic material.

EXAMPLE 4

A series of catalyst samples was prepared generally according to theprocedures of Example 1 to provide an alumina undercoat at a loading of1.0 g/in³ on which a metal oxide topcoat was coated. In the case ofcomparative sample Comp. 1, the topcoat contained no ceria, the topcoatof comparative sample Comp. 2 contained no alumina, and, in a thirdsample in accordance with the present invention, S-3, the topcoatcomprised a ceria-alumina catalytic material containing 46.2 percentceria and 53.8 weight percent alumina. Each of the samples contained 0.5g/ft³ of platinum and had a topcoat loading of about 1.95 g/in³,inclusive of the platinum. In all cases the ceria had a surface area of150 m² /g and the alumina had a surface area of 150 m² /g. The sampleswere tested with the same test gas as described in Example 2 at 275° C.,350° C., 425° C. and 500° C., and the conversion of HC, CO and oxidationof SO₂ to SO₃ at a space velocity of 50,000 was measured. The results ofthese tests are summarized in TABLE III.

                  TABLE III                                                       ______________________________________                                        Inlet Gas   Sample   % C        % C  % C                                      Temp. (°C.)                                                                        No.      SO.sub.2   HC   CO                                       ______________________________________                                        275         Comp. 1  16.3       10.0 96.6                                     275         S-3      0.0        2.4  30.5                                     275         Comp. 2  10.2       0.0  9.4                                      350         Comp. 1  18.9       86.5 99.6                                     350         S-3      4.0        9.8  68.3                                     350         Comp. 2  12.2       6.5  63.1                                     425         Comp. 1  35.5       90.5 99.9                                     425         S-3      11.8       31.6 84.3                                     425         Comp. 2  22.4       18.2 70.4                                     500         Comp. 1  42.2       83.7 99.7                                     500         S-3      12.0       47.4 84.8                                     500         Comp. 2  32.0       31.6 61.0                                     ______________________________________                                    

The data of TABLE III indicate the conversion of hydrocarbons (HC) washighest for sample Comp. 1, containing 100 percent alumina and no ceria,and lowest for sample Comp. 2, containing 100 percent ceria and noalumina. The catalyst in accordance with the present invention, S-3,provided intermediate levels of conversion of HC. Comparable resultswere obtained for conversion of CO at all temperature levels. Theresults of TABLE III concerning the conversion of SO₂ to SO₃ are shownin the perspective-view plot of FIG. 3 from which it is readily seenthat at each temperature level tested a lower degree of conversion ofSO₂ was attained by the S-3 sample in accordance with an embodiment ofthe present invention, than was attained with either the 100 percentalumina (Comp. 1) version or the 100 percent ceria (Comp. 2) version.These data demonstrate that utilizing a ceria-alumina catalytic materialin accordance with the present invention reduces the oxidation of SO₂ ascompared to either a 100 percent ceria or 100 percent alumina catalystcontaining 0.5 g/ft³ of platinum. A series of catalyst compositions wasprepared in order to test catalyst compositions in accordance with thepresent invention against comparative catalyst compositions containingvarious refractory metal oxides and catalytic metals. These catalystswere tested both on a laboratory diagnostic reactor and on dieselengines. The two test engines employed were a Cummins 6BT engine, ratedat 190 horsepower and having a 5.9 liter displacement and a Caterpillar3176 engine, rated at 325 horsepower and having a 10.3 literdisplacement. The operating characteristics of these two engines areshown in TABLE IV based on the operating cycle used to test the catalystcomposition samples.

                  TABLE IV                                                        ______________________________________                                                       Caterpillar 3176                                                                          Cummins 6BT                                        Temperature (°C.)                                                                     Temp. Cycle.sup.a                                                                         Temp. Cycle.sup.a                                  ______________________________________                                        less than 100  0           0                                                  100-200        0           62.6                                               200-300        57.3        36.7                                               300-400        30.9        0.7                                                400-500        11.8        0                                                  Maximum        475         305                                                Temperature (°C.):                                                     ______________________________________                                                   g/HP-hr.sup.b                                                                          Wt. %.sup.c                                                                             g/HP-hr.sup.b                                                                        Wt. % .sup.c                             ______________________________________                                        Particulates:                                                                 VOF        0.036    21.6      0.066  38.4                                     Sulfate    0.005    3.1       0.003  2.0                                      Carbon/Other.sup.d                                                                       0.127    75.3      0.103  59.6                                     Totals     0.168    100.0     0.172  100.0                                    Gas Phase:                                                                    HC         0.123    --        0.300  --                                       CO         3.48     --        1.50   --                                       NO.sub.x   5.06     --        4.34   --                                       ______________________________________                                         .sup.a Percentage of cycle time at which the inlet exhaust to the catalys     lies within the indicated temperature range                                   .sup.b "g/HPhr" = grams per brake horsepowerhour of component emitted in      exhaust                                                                       .sup.c Weight percentage of total particulates provided by the indicated      constituent                                                                   .sup.d "Carbon/Other" values are calculated by difference between the         measured VOF and sulfate components of the exhaust and the total exhaust      particulates. Carbon/Other comprises the dry, solid carbonaceous content      of the particulates plus any water associated with the sulfates. Any          measurement errors will affect the "Carbon/Other" value.                 

As shown in TABLE IV, the Cummins engine runs with a cooler exhaust thandoes the Caterpillar engine and the total engine emissions are roughlycomparable although the Cummins engine runs richer in the volatileorganic fraction (VOF) which is the component most effectively treatedby the diesel oxidation catalyst of the present invention.

EXAMPLE 5

A series of catalyst samples was prepared generally by the methoddisclosed in Example 1 including two catalysts, designated samples S-3and S-3B, comprising embodiments of the present invention and madeexactly in accordance with Example 1 except that for sample S-3Bpalladium was substituted for platinum by using palladium nitrate as thesource of the catalytic metal. Samples S-3 and S-3B each had an aluminaundercoat at a loading of 1.0 g/in³ and a topcoat of the ceria-aluminacoating at a loading of 1.95 g/in³. A series of comparative catalystsdesignated Comp.4, Comp.4M, Comp.4B, Comp.7, Comp.2.3, Comp.6 and Comp.5were made by procedures comparable to those used in Example 1, with thefollowing differences. The comparative catalysts were made without analumina undercoat and, of course, using different refractory metaloxides as indicated by their respective compositions. For the samplescontaining niobia-silica (Comp.4, 4M, 4B and 7) the niobia was providedby dissolving niobium oxalate in the coating slurry. Further, the foamedα-alumina ("FAA") of Comp.2.3 and the silica of other comparativesamples were not ball milled but were dry-jet milled and thenincorporated into the coating step by use of a high speed intensivemixer. The vanadia-titania of sample Comp.6 was incorporated into aslurry containing palladium nitrate as the catalytic metal source.

The silica employed in each case except Comp.2.3 was an extremely poroussilica designated PQ-1022 by its manufacturer, PQ Corporation. ThePQ-1022 silica has a porosity of 1.26 cc/g pore volume comprised ofpores having a radius of from about 10 to 300 Angstroms, and a surfacearea of 225 m² /g. The high porosity of the silica accounts for therelatively low weight loadings of the silica-containing washcoats. Asilica sol was used for the Comp.2.3 sample as described in footnote cof TABLE V. Each of these catalysts, the general composition of which isset forth in TABLE V, was prepared as a slurry of the refractory metaloxide or oxides indicated in TABLE V which had been impregnated with thespecified loading of catalytic metal and then coated onto 400 cpsicylindrical cordierite honeycomb carriers manufactured by NGK andmeasuring 9 inches in diameter by 6 inches in length, providing acatalyst volume of 6.25 liters.

                  TABLE V                                                         ______________________________________                                                             Metal  Hours                                             Catalyst               Loading  Aged                                          Sample   Washcoat      Metal   g/ft.sup.3                                                                           24  100                                 ______________________________________                                        S-3      Ceria-Alumina Pt      0.5    X   X                                   Comp. 4  Niobia-Silica.sup.a                                                                         Pd      50.0   X   X                                   Comp. 4M Niobia-Silica.sup.a                                                                         Pd--Pt  25-5   X   X                                   S-3B     Ceria-Alumina Pd      50.0   X   X                                   Comp. 4B Niobia-Silica.sup.a                                                                         Pt      0.5    X                                       Comp. 7  MnO-Niobia-Silica.sup.b                                                                     Pt      2.2    X                                       Comp. 2.3.sup.c                                                                        Silica-FAA.sup.d                                                                            Pt      2.2    X                                       Comp. 6  Vanadia-Titania.sup.e                                                                       Pd      27     X   X                                   ______________________________________                                         .sup.a The niobiasilica sample catalysts (Comp. 4, 4M and 4B) had             washcoats comprised of 10 percent by weight niobia and 90 percent by          weight silica, with a total washcoat loading of 0.8 g/in.sup.3.               .sup.b The MnOniobia-silica sample catalyst (Comp. 7) had a washcoat          comprised of 90 percent by weight silica, 4 percent by weight niobia and      percent by weight MnO, with a total washcoat loading of 0.6 g/in.sup.3.       .sup.c The silicafoamed alumina sample catalyst (Comp. 2.3) had a washcoa     comprised of 10 percent by weight silica sol binder and 90 percent by         weight of foamed alumina ("FAA"), with a total washcoat loading of 0.6        g/in.sup.3. The alumina has a porosity of 0.0439 cc/g pore volume             comprised of poreshaving a radius of from about 10 to 300 Angstroms, and      surface area of 20.3 m.sup.2 /g.                                              .sup.d "FAA" = foamed alumina                                                 .sup.e The vanadiatitania sample catalyst (Comp. 6) had a washcoat            comprised of 4 percent by weight vanadia and 96 percent by weight titania     with a total washcoat loading of 1.8 g/in.sup.3.                         

All eight sample catalysts were evaluated on the Cummins 6BT engineemploying the U.S. Transient Cycle (commonly, and sometimes hereinbelow,referred to as the "Federal Test Procedure" or "FTP"). A description ofthe U.S. Transient Cycle is set forth in the Code of FederalRegulations, Title 40, Chapter 1, Subpart N, Paragraphs 86: 1310-88 and86: 1312-88, Appendix I(f)(2). The catalyst volume-to-enginedisplacement ratio was 1.06. The catalysts were evaluated for freshactivity (after 24 hours aging) following which the five indicatedsamples were aged for 100 hours and further evaluated. All catalystswere aged on a 1986 Cummins NTC diesel engine rated at 400 horsepowerand having a 14.0 liter displacement. The aging cycle employed flowedthe engine exhaust through three catalysts of 6.25 liter volume each,simultaneously and in parallel, with the engine load adjusted to providefifteen minute cycles during which the exhaust attained inlettemperatures as follows for the indicated amount of time:

330-400° C. for 14% of the time,

400-500° C. for 22% of the time,

500-550° C. for 50% of the time, and

550-565° C. for 14% of the time.

The S-3 and S-3B samples each contain 46.2 weight percentaluminum-stabilized ceria and 53.8 weight percent alumina.

TABLE VI shows the results of the fresh (aged 24 hours) catalyst samplestested under the Federal Test Procedure on the Cummins 6BT engine withall recorded exhaust emissions being given in grams per brakehorsepower-hour. All emissions are measured quantities except for"Carbon+Other" which is calculated by difference. The measured valuesare the average of four different runs conducted under the Federal TestProcedure which were carried out over the space of two days in order toaccount for day-to-day variations. TABLE VI also shows the base linevalues of the diesel exhaust operated without catalytic treatment overan average of 24 runs. The difference between the runs carried outwithout catalytic treatment and the runs carried out using the variouscatalyst samples were utilized to calculate the percent conversion ofeach of the emissions components. The percent conversion is thepercentage of the emissions contained in the untreated exhaust whichwere converted to innocuous components by utilization of the catalystsamples. The abbreviation "TPM" is used for "total particulate matter".

                                      TABLE VI                                    __________________________________________________________________________    Catalyst                        Carbon +                                      Sample                                                                              HC  CO NOx TPM VOF  Sulfate                                                                             Other                                         __________________________________________________________________________    None - Untreated engine exhaust                                               Grams.sup.a                                                                         0.299                                                                             1.5                                                                              4.34                                                                              0.172                                                                             0.0611                                                                             0.0034                                                                              0.108                                         S-3                                                                           Grams.sup.a                                                                         0.188                                                                             1.11                                                                             4.3 0.118                                                                             0.0256                                                                             0.0016                                                                              0.0908                                        % C.sup.b                                                                           37.4                                                                              26 0.96                                                                              31.7                                                                              58.1 53.1  15.9                                          Comp. 4                                                                       Grams 0.198                                                                             1.28                                                                             4.22                                                                              0.123                                                                             0.0272                                                                             0.0022                                                                              0.0936                                        % C   34.1                                                                              14.9                                                                             2.7 28.8                                                                              55.4 37    13.3                                          Comp. 4M                                                                      Grams 0.213                                                                             1.34                                                                             4.22                                                                              0.123                                                                             0.0302                                                                             0.0025                                                                              0.0903                                        % C   29.1                                                                              11 2.8 28.8                                                                              50.6 28.2  16.4                                          S-3B                                                                          Grams 0.155                                                                             1.31                                                                             4.31                                                                              0.118                                                                             0.0258                                                                             0.0025                                                                              0.0897                                        % C   48.3                                                                              13 0.73                                                                              31.7                                                                              57.7 26    16.9                                          Comp. 4B                                                                      Grams 0.208                                                                             1.17                                                                             4.27                                                                              0.128                                                                             0.0359                                                                             0.0033                                                                              0.0888                                        % C   30.7                                                                              22.2                                                                             1.5 25.9                                                                              41.2 4.8   17.8                                          Comp. 7                                                                       Grams 0.198                                                                             1.09                                                                             4.31                                                                              0.135                                                                             0.0378                                                                             0.0028                                                                              0.0944                                        % C   34.1                                                                              27.2                                                                             0.79                                                                              21.5                                                                              38.1 17.2  12.6                                          Comp. 2.3                                                                     Grams 0.185                                                                             1.09                                                                             4.34                                                                              0.135                                                                             0.0306                                                                             0.0049                                                                              0.0995                                        % C   38.2                                                                              27.5                                                                             0.1 21.5                                                                              38.1 -43.6 7.9                                           Comp. 6                                                                       Grams 0.135                                                                             1.51                                                                             4.35                                                                              0.118                                                                             0.0255                                                                             0.003 0.0895                                        % C   54.9                                                                              -0.3                                                                             -0.25                                                                             31.7                                                                              58.3 11.4  17.1                                          __________________________________________________________________________     .sup.a Grams per brake horsepowerhour                                         .sup.b "% C" means the percentage conversion of the indicated constituent     A negative % C means the treated exhaust contained more of the constituen     than did the untreated exhaust.                                          

The results tabulated in TABLE VI indicate that with respect to VOFconversion and total particulates conversion, the best results wereobtained by S-3, S-3B and Comp.6 catalysts, with the Comp.4 samplegiving the next best results. As to sulfate emissions, the Comp.2.3sample exhibited sulfate emissions which were greater than those of theuntreated exhaust, all the other samples tested giving at least somereduction in sulfates as compared to the untreated exhaust. This findingis consistent with the relatively low temperature of the Cummins 6BTengine. With respect to gas phase emissions (HC, CO and NO_(x)) Comp.6,S-3B and Comp.5 gave the best HC reduction while Comp.2.3, Comp.7 andS-3 gave the best CO conversion. There was little catalytic effect onNO_(x) emissions as one would expect in the relatively oxygen-richenvironment of a diesel exhaust.

EXAMPLE 6

As indicated in TABLE V, five of the catalysts tested were then aged toa total of 100 hours and re-evaluated on the Cummins 6BT engine. Theresults of the evaluation of the 100-hour aged samples are summarized inTABLE VII.

                                      TABLE VII                                   __________________________________________________________________________    Catalyst                        Carbon +                                      Sample                                                                              HC  CO NOx TPM VOF  Sulfate                                                                             Other                                         __________________________________________________________________________    None - Untreated engine exhaust                                               Grams.sup.a                                                                         0.305                                                                             1.55                                                                             4.46                                                                              0.179                                                                             0.0675                                                                             0.0039                                                                              0.108                                         S-3                                                                           Grams.sup.a                                                                         0.188                                                                             1.27                                                                             4.31                                                                              0.123                                                                             0.0284                                                                             0.0018                                                                              0.0928                                        % C.sup.b                                                                           38.4                                                                              17.9                                                                             3.3 31.3                                                                              57.9 53.8  14.1                                          Comp. 4                                                                       Grams 0.218                                                                             1.47                                                                             4.37                                                                              0.128                                                                             0.0327                                                                             0.0023                                                                              0.093                                         % C   28.5                                                                              4.9                                                                              1.9 28.5                                                                              51.6 41    13.9                                          Comp. 4M                                                                      Grams 0.238                                                                             1.49                                                                             4.37                                                                              0.13                                                                              0.0349                                                                             0.0031                                                                              0.092                                         % C   22  3.6                                                                              1.9 27.4                                                                              48.3 20.5  14.8                                          S-3B                                                                          Grams 0.175                                                                             1.27                                                                             4.38                                                                              0.12                                                                              0.0282                                                                             0.0022                                                                              0.0896                                        % C   42.6                                                                              17.9                                                                             1.7 33  58.2 43.6  17                                            Comp. 6                                                                       Grams 0.22                                                                              1.69                                                                             4.42                                                                              0.14                                                                              0.0308                                                                             0.0042                                                                              0.105                                         % C   27.9                                                                              -9.3                                                                             0.8 21.8                                                                              54.4 -7.7  2.8                                           __________________________________________________________________________     .sup.a Grams per brake horsepowerhour                                         .sup.b "% C" means the percentage conversion of the indicated constituent     A negative % C means the treated exhaust contained more of the constituen     than did the untreated exhaust.                                          

Table VII shows that the best results were attained by the S-3 and S-3Bcatalysts for both total particulate emissions and VOF conversion. Withrespect to HC reduction the best performance was shown by S-3B althoughthe S-3 catalyst proved to be the most stable, the results attained bythe S-3 catalyst after 100 hours aging being actually better than thoseattained by the 24-hour aged S-3 sample. The S-3B catalyst exhibitedimproved CO conversion for the 100-hour aged catalyst as compared to thefresh (24-hour aged) catalyst. Note that the Comp.6 sample removedessentially no CO at 24 hours and became a net CO producer after beingaged for 100 hours. The results of TABLE VI and VII clearly show thatthe catalyst compositions of the present invention, S-3 and S-3B, gavethe best overall emissions control and the best durability as evidencedby 100 hours of aging.

EXAMPLE 7

In order to compare the effect of different catalytic metal loadings onthe performance of catalysts in accordance with the present invention,three sample catalysts in accordance with the present invention wereprepared in accordance with the procedure of Example 1. Thus, eachcatalyst comprised a cordierite 400 cpsi substrate containing 1.95 g/in³of the ceria-alumina catalytic material of the invention. Theceria-alumina catalytic material contained 46.2 weight percent ofaluminum-stabilized ceria and 53.8 weight percent of activated alumina.Each catalyst had an alumina undercoat in the amount of 1.00 g/ft³ ontowhich the ceria-alumina catalytic material was coated. One sample,designated S-3.5Pt had 0.5 g/ft³ of platinum dispersed thereon, anothersample, designated S-3.20Pt had 2.0 g/ft³ of platinum dispersed thereonand a third sample, designated S-3Pd had 50 g/ft³ of palladium dispersedthereon. Each catalyst was tested under the Federal Test Procedure totreat an exhaust generated by a Cummins C-series 250 HP diesel enginehaving a displacement of 8 liters, so that a catalyst volume-to-enginedisplacement ratio of 0.78 was utilized. The effectiveness of the samplecatalyst was tested in the same manner as that of Example 6 and theresults with respect to conversion of total particulates (TPM) andgaseous phase HC and CO are set forth in TABLE VIII.

                  TABLE VIII                                                      ______________________________________                                                 % C.sup.a     % C.sup.a                                                                             % C.sup.a                                      Sample   TPM           HC      CO                                             ______________________________________                                        S-3.5Pt  47            28      7.5                                            S-3.20Pt 48            69      74                                             S-3Pd    48            52      35                                             ______________________________________                                         .sup.a "% C" means the percentage conversion of the indicated constituent                                                                              

The data of TABLE VIII show that all three samples were nearly identicalwith respect to the percentage conversion of total particulates althoughthe larger loadings of catalytic metal made a dramatic difference in thepercentage conversions of the gaseous HC and CO. These results areconsistent with the data of Example 6 and TABLE VII, from which it willbe noted that S-3 and S-3B gave substantially similar results withrespect to total particulates reduction in spite of the fact that S-3contains only 0.5 g/ft³ of platinum and S-3B contains 50 g/ft³ ofpalladium. The lack of pronounced effect on total particulate reductionbetween a catalyst containing 100 times more platinum group metal thananother, strongly suggests the irrelevancy of the presence of thecatalytic metal insofar as total particulate reduction is concerned, andthat particulate reduction is attained by the effect of ceria-aluminacatalytic material.

EXAMPLE 8

In order to further demonstrate the irrelevancy of the platinum metalloading insofar as catalytic activity of the ceria-alumina catalyticmaterial with respect to total particulate reduction is concerned, aseries of samples of catalytic material powder was prepared. This wasdone by utilizing the ceria-alumina washcoat material of Example 7containing various quantities of platinum metal ranging from 0 to theequivalent of 5.0 g/ft³ of platinum if the washcoat were to be coatedupon a 400 cpsi NGK cordierite substrate. The resultant series ofpowders were each mixed with 10 weight percent of a diesel enginelubricating oil, Cummins SAE-15W Premium Blue Diesel Engine Lube Oil,and the sample of the mixture was evaluated by simultaneousthermogravimetric analysis and differential thermal analysis (TGA/DTA)for combustion of the lubricating oil. It should be noted that unburneddiesel engine lubricating oil constitutes a significant portion of thevolatile organic fraction (VOF) of diesel exhaust particulate emissionsand the efficacy of the ceria-alumina catalytic material in catalyzingcombustion of the lubricating oil is a good indication of theeffectiveness of the ceria-alumina catalytic material in catalyzingoxidation of VOF and, thereby, reduction of particulate emissions.Thermogravimetric analysis measures the weight gain or loss of a sample(indicating a chemical reaction undergone by the sample) as a functionof the temperature to which the sample is heated. Differential thermalanalysis measures the amount of energy (heat) absorbed by the sample(indicating that the sample has undergone an endothermic reaction) orliberated by the sample (indicating that the sample has undergone anexothermic reaction) as a function of the temperature to which thesample is heated. FIG. 4 is a plot of the results obtained by heatingthe mixture of catalytic material powder and lubricating oil in atemperature regime ranging from ambient temperature to 600° C. andrecording the TGA/DTA data. The DTA peak area was corrected for theweight change determined by the TGA so that the results attained areproportional to the amount of lubricating oil combusted, i.e., to theeffectiveness of the tested ceria-alumina catalytic materials, which areidentical except for the varying platinum metal loadings. The resultsattained are plotted in FIG. 4 wherein, despite some experimentalscatter in the data points, the trend line indicates substantially noeffect of the platinum content of the catalytic material insofar aslubricating oil combustion is concerned. Thus, about the same proportionof combustion was attained for the ceria-alumina catalytic materialcontaining no platinum as for that containing incremental amounts ofplatinum up to and including the equivalent of 5 g/ft³ on a 400 cpsicarrier.

EXAMPLE 8A

An equivalent test of silica based and silica-niobia based refractorymetal oxide powders on which varying amounts of platinum were dispersedwas carried out. Those test results showed that the ceria-aluminacatalytic material of the present invention provided better performancefor lubricating oil combustion as measured by DTA and therefore, byimplication, for catalytic oxidation of VOF in diesel engine exhaust.

EXAMPLE 9

S-3 and comparative Comp.4 catalyst samples were tested on the exhaustof the Caterpillar 3176 engine. As previously noted, this engine runswith a considerably hotter exhaust than the Cummins 6BT engine and testcatalysts of the same size (9 inches by 6 inches providing a catalystvolume of 6.25 liters) were tested on this larger engine, providing acatalyst volume-to-engine displacement ratio of 0.607. S-3 and Comp.4catalyst samples were aged for 50 hours on an aging cycle similar tothat described in Example 5 but from a minimum of about 300° C. to amaximum of about 530° C.

The results of this test are shown in TABLE IX as the average of sixhot-start runs in accordance with the Federal Test Procedure.

                                      TABLE IX                                    __________________________________________________________________________    Catalyst                        Carbon +                                      Sample                                                                              HC  CO NOx TPM VOF  Sulfate                                                                             Other                                         __________________________________________________________________________    None - Untreated engine exhaust                                               Grams.sup.a                                                                         0.123                                                                             3.48                                                                             5.06                                                                              0.168                                                                             0.0363                                                                             0.0052                                                                              0.1265                                        S-3                                                                           Grams.sup.a                                                                         0.1566                                                                            2.5                                                                              4.95                                                                              0.138                                                                             0.0213                                                                             0.0039                                                                              0.1128                                        % C.sup.b                                                                           54  28.2                                                                             2.2 17.9                                                                              41.3 25    10.8                                          Comp. 4                                                                       Grams 0.0833                                                                            1.76                                                                             5.02                                                                              0.177                                                                             0.017                                                                              0.0217                                                                              0.1383                                        % C   32.3                                                                              20.7                                                                             0.8 -5.4                                                                              53.2 -317  -9.3                                          __________________________________________________________________________     .sup.a Grams per brake horsepowerhour                                         .sup.b "% C" means the percentage conversion of the indicated                 constituents. A negative % C means the treated exhaust contained more of      the constituent than did the untreated exhaust.                          

The results summarized in TABLE IX show that the S-3 catalyst reducedtotal particulate emissions by 17.9 percent and VOF by 41.3 percentwhereas the Comp.4 sample, although it gave a higher VOF reduction at53.2 percent, resulted in an increase of total particulate emissions,because of its very high sulfate make which resulted in sulfateemissions 317 percent higher than those emitted in the untreatedexhaust. The tendency of the Comp.4 sample to produce large amounts ofsulfate in the hot exhaust environment of the Caterpillar 3176 enginestands in marked contrast to the efficiency of the S-3 catalyst inattaining a 25 percent reduction in sulfate emissions and therefore anoverall reduction in total particulates. The fact that the S-3 catalystexhibited lower total particulates and VOF removal levels on theCaterpillar 3176 engine than on the Cummins 6BT engine is attributableto the smaller catalyst volume relative to engine size encountered onthe Caterpillar engine test and to the fact that the concentration ofVOF, the component most vigorously treated by the catalyst, is some 40percent lower in the exhaust of the Caterpillar engine than in theexhaust of the Cummins engine.

EXAMPLE 10

In order to compare the effect on conversion of SO₂ to SO₃, and thussulfate-make of a catalyst, three comparative samples, one of which(designated Comp.11) is a commercially available diesel exhaustcatalyst, and each containing high platinum group metal loadings, werecompared to a fourth sample comprising an embodiment of the presentinvention. Three samples, Comp.1, Comp.2 and S-3, were preparedgenerally in accordance with the procedure of Example 1 to coatcylindrical carriers comprising 400 cpsi cordierite cores measuring 1.5inches in diameter by 3 inches in length. The samples were aged for 10hours at 500° C. by having a mixture of 10 percent steam in air flowedthrough each sample. Comparative sample Comp.1 comprised 50 g/ft³ ofplatinum disposed on an activated alumina carrier and comparative sampleComp.2 had a 50 g/ft³ platinum group metal loading, the platinum groupmetal comprising platinum and rhodium in a 5:1 weight ratio disposed ona ceria-alumina catalytic material comprising 53.8 percent by weightalumina and 46.2 percent by weight aluminum-stabilized ceria. The S-3sample comprised, in accordance with one embodiment of the presentinvention, 0.5 g/ft³ of platinum dispersed on a ceria-alumina catalyticmaterial comprising 46.2 percent by weight aluminum-stabilized ceria and53.8 percent by weight alumina with one-half the platinum metaldispersed on the aluminum-stabilized ceria and one-half the platinummetal dispersed on the alumina. The commercially available catalyst fordiesel exhaust applications was analyzed and found to comprise acatalytic material dispersed on a honeycomb-type carrier having 400cells per square inch. The commercial catalyst contained about 50 g/ft³of platinum dispersed on a support comprised primarily of titania,vanadia and alumina. A core 2.5 inches long and 1.5 inches in diameterwas cut from the commercial catalyst and this comparative catalyst corewas designated as Comp.11. The four catalyst samples were tested atspace velocities of 50,000 and 90,000 at temperatures of 275° C., 350°C., 425° C. and 500° C. (In this Example 10 and in Example 11 below, theflow rate of the reaction gas was adjusted as necessary to compensatefor the slight difference in catalyst volume so that each tested samplewas evaluated at the same space velocity.) Each sample was held at theindicated temperature for 10 minutes during the evaluation. The test gasused in the laboratory diagnostic unit comprised 10 percent steam, 10percent oxygen, 4.5 percent CO₂, 1000 ppm NO, 28.57 ppm heptane(equivalent to 200 ppm C₁ hydrocarbons), 28.6 ppm CO, 50 ppm SO₂,balance nitrogen. (The percents are volume percents and "ppm" meansparts per million by volume.) The results of these evaluations are givenin TABLE X.

                  TABLE X                                                         ______________________________________                                                 Percent Conversion at                                                Catalyst Indicated Space Velocity                                             Sample/  50,000 SV      90,000 SV                                             Inlet Temp.                                                                            CO     HC       SO.sub.2                                                                           CO     HC   SO.sub.2                            ______________________________________                                        Comp. 1                                                                       275° C.                                                                         99.5   68.6     56.9 94.9   52.4 29.4                                350° C.                                                                         99.5   83.0     76.9 96.5   70.5 58.5                                425° C.                                                                         100    87.4     94.3 --     --   --                                  500° C.                                                                         100    89.0     92.2 --     --   --                                  Comp. 2                                                                       275° C.                                                                         100    52.5     --   98.5   47.6 8.0                                 350° C.                                                                         99.0   77.5     11.8 96.9   61.7 9.8                                 425° C.                                                                         99.0   84.2     31.4 96.0   74.4 23.5                                500° C.                                                                         98.1   90.7     47.1 95.5   73.2 37.3                                Comp. 11                                                                      275° C.                                                                         97.1   16.7     0.0  84.7   4.8  2.0                                 350° C.                                                                         99.0   54.5     2.0  93.0   41.0 2.0                                 425° C.                                                                         99.5   75.0     23.6 97.0   63.2 15.7                                500° C.                                                                         99.5   85.9     54.9 97.4   73.2 38.0                                S-3                                                                           275° C.                                                                         30.5   2.4      0.0  10.9   0.0  0.0                                 350° C.                                                                         68.3   9.8      4.0  52.4   9.5  0.0                                 425° C.                                                                         84.3   31.6     11.8 59.5   24.3 3.9                                 500° C.                                                                         84.4   47.4     12.0 60.0   28.6 4.1                                 ______________________________________                                    

The data of TABLE X shows that the comparative samples Comp.1 and Comp.2exhibit very high conversion of SO₂ to SO₃, and thus high sulfate make,even at the lowest test temperature of 275° C. and high space velocityof 90,000. Although comparative sample Comp.2 exhibits less sulfate makethan Comp.1 (but significantlly more than catalyst S-3, discussedbelow), this is believed to be due primarily to the modifying effect ofrhodium on the SO₂ oxidation activity of platinum. The Comp.2 catalysthas the economic disadvantage of being too costly because of the veryhigh cost of rhodium even as compared to the cost of platinum. Bothcomparative samples Comp.1 and Comp.2 show high HC and CO conversion.S-3, the sample in accordance with an embodiment of the presentinvention, exhibits greatly reduced SO conversion relative to bothComp.1 and Comp.2 with practically no SO conversion occurring in the lowtemperature regime and with relatively small SO₂ conversion even at thehigh temperature regime. Some activity for conversion of gaseous HC andCO is exhibited by catalyst S-3, especially at the high temperatureregime where good CO and moderate HC activity is seen. The data of TABLEX thus clearly demonstrate that the utilization of a low platinumloading on the ceria-alumina catalytic material of the present inventionprovides excellent control of SO₂ oxidation and consequently excellentcontrol of total particulates emission in a diesel engine exhaust. Itshould be noted that the diagnostic test is a very stringent test ofsulfate oxidation as compared to actual engine performance. Experiencehas shown that a given catalyst will perform better with respect tosulfate oxidation in treating an actual diesel engine exhaust than itwill in the diagnostic engine test.

The comparative catalyst sample Comp.11 is seen to suppress SO₂oxidation in a manner comparable to that of sample S-3, but only up to atemperature between 350° and 425° C. At 425° C. and higher temperaturesthe Comp.11 sample exhibits greatly increased SO₂ oxidation as comparedto the S-3 catalyst sample. Accordingly, the catalyst sample of thepresent invention, even with a 0.5 g/ft³ platinum loading, appears to besignificantly better with respect to SO₂ oxidation in higher temperatureregimes than the commercial catalyst of Comp.11. The comparative samplesComp.1, Comp.2 and Comp.11 all contain high platinum loadings andconsequently show higher HC and CO conversion than does the 0.5 g/ft³platinum S-3 catalyst sample. However, as pointed out elsewhere herein,HC and CO emissions generally satisfactorily controlled by engine designand the problem which the art is seeking to over-come is to control thetotal particulates emissions which, as noted above, is in part afunction of sulfate make. The catalysts of the present invention, withno or a very low loading of platinum, show excellent activity forreducing total particulate emissions because of their unexpected goodactivity for oxidizing VOF and their low sulfate make. Further, it isobviously economically advantageous to eliminate or drastically reducethe platinum metal loading in accordance with the teachings of thepresent invention.

EXAMPLE 11

A catalyst sample in accordance with an embodiment of the presentinvention was prepared and designated sample S-3P. Catalyst sample S-3Pis identical to catalyst sample S-3 of Example 10 except that itcontains a platinum loading of 2.0 g/ft³. The S-3P catalyst sample was3.0 inches long by 1.5 inches in diameter. Catalyst S-3P was tested bypassing therethrough a test gas in the same manner as described inExample 10 at space velocities of 50,000 and 90,000 at temperatures of275° C., 350° C., 425° C. and 500° C. The results of this test are shownin TABLE XI.

                  TABLE XI                                                        ______________________________________                                                 Percent Conversion at                                                Catalyst Indicated Space Velocity                                             Sample/  50,000 SV      90,000 SV                                             Inlet Temp.                                                                            CO     HC       SO.sub.2                                                                           CO     HC   SO.sub.2                            ______________________________________                                        S-3P                                                                          275° C.                                                                         99.0   10.0     16.0 88.5   2.6  2.0                                 350° C.                                                                         100    87.8     21.6 98.0   74.1 5.9                                 425° C.                                                                         100    90.5     33.3 98.5   82.7 19.2                                500° C.                                                                         99.5   83.1     35.3 98.1   73.3 31.4                                ______________________________________                                    

TABLE XI shows, as would be expected, that the S-3P catalyst exhibitshigher SO₂ oxidation at all temperature levels and space velocities ascompared to the S-3 catalyst of Example 10 which contains 0.5 g/ft³ ofplatinum, one-fourth of the amount of platinum (2.0 g/ft³) which S-3Pcontains. However, the S-3P sample also exhibited higher HC and COconversions, which shows that a modest increase in platinum loading,still keeping the total platinum loading to very low levels as comparedto prior art catalysts, can accommodate a higher HC and CO conversionbut at the potential cost of somewhat increased particulate emissionsbecause of additional sulfate make. However, in certain circumstances itmay be desirable to attain the higher HC and CO conversions attainablewith the catalyst of the present invention by a modest increase inplatinum loading.

EXAMPLE 12

In order to evaluate the effect of ceria in the catalyst composition ofthe present invention, a comparative sample, Comp.1 of Example 4, wasprepared generally in accordance with the procedure of Example 1 butomitting the ceria component of the catalytic material. Thus, theresulting catalyst comprised an activated alumina washcoat having 0.5g/ft³ of platinum disposed thereon. This sample designated Comp.3C wassubjected to the same test as in Examples 10 and 11 and the resultsthereof are summarized in TABLE XII and show that the SO₂ conversionover this catalyst is significantly greater than for the S-3 catalyst,especially at low temperatures. Higher conversions of HC and CO werealso attained. This data clearly indicate that the ceria plays animportant modifying role in the oxidation activity of the platinum.

                  TABLE XII                                                       ______________________________________                                                 Percent Conversion at                                                Catalyst Indicated Space Velocity                                             Sample/  50,000 SV      90,000 SV                                             Inlet Temp.                                                                            CO     HC       SO.sub.2                                                                           CO     HC   SO.sub.2                            ______________________________________                                        Comp. 3C                                                                      275° C.                                                                         96.6   10.0     16.3 85.4   4.9  4.6                                 350° C.                                                                         99.6   86.5     18.9 95.0   57.4 12.6                                425° C.                                                                         99.9   90.5     35.5 98.1   74.0 33.7                                500° C.                                                                         99.7   83.7     42.2 98.3   74.0 33.7                                ______________________________________                                    

EXAMPLE 13

A catalyst was prepared in accordance with the present inventiongenerally following the teachings of Example 1, except that no aluminaundercoat was utilized. Thus, this sample comprised 1.95 g/ft³ of aceria-alumina catalytic material containing 46.2 weight percentaluminum-stabilized ceria (164 m² /g BET surface area) and 53.8 weightpercent alumina (150 m² /g BET surface area) disposed directly upon thecarrier without an alumina under- coat, and having 0.5 g/ft³ of platinumdispersed thereon. This catalyst, designated S-3SC was aged and testedin the same manner as in Example 10 and the results thereof are shown inTABLE XIII. The performance of this sample is seen to be essentially thesame as that of S-3 (Example 10, TABLE X) for the gas phase reactions,indicating that the presence of the alumina undercoat is not essentialwith respect to either low sulfate make or HC and CO oxidation.

                  TABLE XIII                                                      ______________________________________                                                 Percent Conversion at                                                Catalyst Indicated Space Velocity                                             Sample/  50,000 SV      90,000 SV                                             Inlet Temp.                                                                            CO     HC       SO.sub.2                                                                           CO     HC   SO.sub.2                            ______________________________________                                        S-3SC                                                                         275° C.                                                                         25.4   0.0      2.0  31.0   0.0  0.0                                 350° C.                                                                         71.9   11.9     5.9  62.5   15.8 4.1                                 425° C.                                                                         85.6   28.9     9.8  78.7   29.3 5.9                                 500° C.                                                                         86.3   48.7     20.4 76.1   42.5 10.7                                ______________________________________                                    

EXAMPLE 14

A. Catalysts were prepared generally in accordance with the proceduresof Example 1 to provide a series of three otherwise identicalcompositions containing a ceria-alumina catalytic material in accordancewith the teachings of the present invention having platinum dispersedthereon, including 0.0, 0.5 and 2.0 g/ft³ platinum. Each catalystcomprised a γ-alumina undercoat at a loading of 1.0 g/in³ upon which wascoated a top coat layer comprised of 1.05 g/in³ γ-alumina plus 0.90g/in³ alumina-stabilized ceria (2.5 weight percent Al₂ O₃ based on thecombined weight of bulk ceria and alumina dispersed therein). Thecatalysts were coated onto a 9 inch diameter by 6 inch long, 400 cpsicordierite substrate. The resulting catalyst samples were designated asS-4 (0.0 g/ft³ platinum, aged 24 hours), S-5 (0.5 g/ft³ platinum, aged25 hours) and S-6 (2.0 g/ft³ platinum, aged 24 hours).

B. The three catalyst samples were conditioned prior to evaluation usingan aging cycle involving 20 minutes each at Modes 2,6 and 8 of theEuropean 13 Mode Test Procedure (ECE R.49 Thirteen Mode Cycle). ThisTest Procedure is set forth in the Society of Automotive EngineersPublication, SAE Paper #880715, published at the International Congressand Exposition, Detroit, Mich. Feb. 29 through Mar. 4, 1988, by GeorgioM. Cornetti et al. The disclosure of this SAE publication isincorporated by reference herein. Prior to testing to develop the dataof TABLE XV and FIGS. 5-8, the three catalyst samples were aged 24 or 25hours as indicated below on a Cummins 6BT turbocharged diesel enginehaving a 5.9 liter displacement and rated at 190 horsepower. For bothaging and test purposes, the engine was run with low sulfur fuel (0.05weight percent sulfur) under steady state conditions using test modesselected from the aforesaid European 13 Mode Cycle Test Procedure.

The engine conditions for the test modes along with average (for fiveruns) catalyst inlet temperatures and baseline emissions (of untreatedengine exhaust) are shown in TABLE XIV.

                  TABLE XIV                                                       ______________________________________                                        Cummins 6BT 190 HP Turbocharged                                               Diesel Engine, 5.9 Liter Displacement,                                        Conditions For Steady State Catalyst Tests                                    ______________________________________                                        Engine Conditions                                                                                          Average                                          Test                  %      Catalyst Inlet                                   Mode No.   rpm        Load   Temp. (°C.)                               ______________________________________                                        8          2515       100    571 ± 2                                       10         2515       50     338 ± 4                                       6          1609       100    549 ± 5                                       4          1609       50     400 ± 4                                       2          1560       10     214 ± 3                                       1          803        Low     128 ± 16                                     ______________________________________                                        Baseline Emissions - Untreated Exhaust                                        Test         Average Emissions (g-bhp-hr).sup.1                               Mode No.     TPM    SOF        HC   CO                                        ______________________________________                                        8            0.097  0.010      0.122                                                                              0.46                                      10           0.151  0.047      0.212                                                                              0.68                                      6            0.221  0.016      0.099                                                                              2.23                                      4            0.146  0.023      0.103                                                                              0.52                                      2            0.265  0.137      0.541                                                                              2.57                                      1            --     0.078      1.04 3.01                                      ______________________________________                                         .sup.1 grams per brake horsepower hour                                   

The conditioned and aged catalyst samples S-4, S-5 and S-6 were testedfor conversion of emission components in diesel exhaust generated by thetest engine used to generate the data of Table XIV, as a function ofsteady state test mode and catalyst inlet temperature, i.e., thetemperature of the diesel engine exhaust introduced to the catalyst. Theresults are summarized in TABLE XV.

                  TABLE XV                                                        ______________________________________                                        Sample/                                                                       (Pt Load                                                                              Test      Cat. Inlet                                                                              % Removal                                         g/ft.sup.3)                                                                           Mode No.  Temp. °C.                                                                        SOF  TPM   HC   CO                                ______________________________________                                        S-4     2         209       72   63    31   1                                 (0.0)   10        335       60   27    32   7                                         4         399       62   18    38   18                                        6         547       84   -40   44   27                                        8         572       79   -181  39   -4                                S-5     2         215       60   45    27   6                                 (0.5)   10        343       58   28    41   63                                        6         549       91   -64   56   85                                        8         570       80   -201  62   45                                S-6     1         127       56   52    37   -1                                (2.0)   2         215       61   61    39   8                                         10        341       53   31    74   86                                        4         397       61   22    82   87                                        6         554       89   -60   78   95                                        8         572       79   -200  71   70                                ______________________________________                                    

The data of TABLE XV show that all three catalysts are comparable in SOFremoval performance as a function of temperature, with the catalystcontaining no platinum (S-4) performing as well as the catalystscontaining platinum (S-5 and S-6).

With reference to TABLE XV and FIGS. 5-8, it is seen that the SOFremoval performance as a function of inlet temperature of the threecatalysts are compared in FIG. 5. As can be seen, all three samples arecomparable across the temperature range of about 120°to 575° C. with thesample containing no platinum (S-4) performing as well as or betterthan, the platinum-containing samples S-5 and S-6.

The SOF removal performance is also reflected in the total particulate(TPM) removal levels of the three catalysts which are compared in FIG.6. The platinum-free catalyst sample is comparable to, or better than,the platinum-containing catalyst samples S-5 and S-6 at alltemperatures. Note also, all three catalyst samples make particulates atthe highest temperatures of the test. This is due to sulfate-make fromthe oxidation of gas phase SO₂ to SO₃ Thus, even the platinum-freesample makes sulfate at extremely high temperatures, but apparently to aslightly lesser extent than the platinum-containing samples, reflectingthe lower gas phase activity of the platinum-free sample.

The gas phase activity of the three catalyst samples are compared inFIGS. 7 and 8 for, respectively, hydrocarbon ("HC") and carbon monoxide("CO") gas phase conversions. Although the platinum-free sample S-4exhibits some gas phase activity for HC and CO conversion, it is clearfrom these results that the platinum-containing samples S-5 and S-6 havesubstantially higher gas phase activity. This is especially clear in thecase of CO conversion. The platinum-free sample S-4 has some gas phaseactivity because the ceria component has activity as an oxidationcatalyst.

These results show quite well the surprising finding that aplatinum-free catalyst in accordance with the present invention exhibitsvery good particulates removal from diesel engine exhaust because of itsactivity for the removal and combustion of VOF, and that the preciousmetal is not needed to accomplish this function. If there is a need toenhance gas phase HC and CO activity, this can be accomplishedseparately by adding a limited amount of platinum to the catalyst.

While the invention has been described in detail with respect tospecific preferred embodiments thereof it will be appreciated thatvariations thereto may be made which nonetheless lie within the scope ofthe invention and the appended claims.

What is claimed is:
 1. An oxidation catalyst composition comprises arefractory carrier on which is disposed a coating of a ceria-aluminacatalytic material consisting essentially of a combination of ceriahaving a BET surface area of at least about 10 m² /g and alumina havinga BET surface area of at least about 10 m² /g, in the absence of aplatinum group metal.
 2. The catalyst composition of claim 1 wherein theceria and alumina each comprises from about 5 to 95 percent by weight ofthe combination.
 3. The catalyst composition of claim 1 wherein theceria and alumina each comprises from about 10 to 90 percent by weightof the combination.
 4. The catalyst composition of claim 1, claim 2 orclaim 3 wherein the ceria and the alumina are each disposed inrespective discrete layers, one overlying the other.
 5. The catalystcomposition of claim 1, claim 2 or claim 3 wherein the ceria comprisesan aluminum-stabilized ceria.
 6. The catalyst composition of claim 1wherein the ceria and alumina each comprises from about 40 to 60 percentby weight of the combination.
 7. The catalyst composition of claim 1wherein the ceria and the alumina each has a BET surface area of fromabout 25 m² /g to 200 m² /g.
 8. A catalyst composition for purifyingdiesel engine exhaust comprises a refractory carrier on which isdisposed a coating of a catalytic material consisting essentially of acombination of ceria having a BET surface area of at least about 10 m²/g and alumina having a BET surface of at least about 10 m² /g, theceria and alumina each comprising from about 5 to 95 percent by weightof the combination, in the absence of a platinum group metal.
 9. Thecatalyst composition of claim 8 wherein the ceria and alumina eachcomprises from about 10 to 90 percent by weight of the combination. 10.The catalyst composition of claim 8 wherein the ceria and the aluminaeach comprises from about 40 to 60 percent by weight of the combination.11. The catalyst composition of claim 8, wherein the ceria comprisesaluminum-stabilized ceria.
 12. The catalyst composition of claim 8,claim 9 or claim 10 wherein the ceria and the alumina are each disposedin respective discrete layers, one overlying the other.